This invention relates to an improved process for the recovery of caprolactam and medium-to-high pressure (usable) steam from a caprolactam forming process. In particular, this invention relates to an energy efficient caprolactam purification process whereby steam recovered in one part of the process may be re-used to operate equipment in other parts of the process.
There are a number of methods known in the art for producing and purifying caprolactam. U.S. Pat. No. 2,813,858 (Joris) discusses the well known Beckman rearrangement. U.S. Pat. No. 2,813,858 also mentions methods such as hydration of adipodinitrile and reduction combined with the rearrangement of nitrocyclohexane which can also be used to prepare reaction mixtures from which lactam is recoverable, e.g., by distillation. Another source of caprolactam, as discussed in U.S. Pat. No. 5,495,014 (Fuchs et al.), involves cleaving oligomers and polymers comprised essentially of the lactam repeating unit.
U.S. Pat. No. 5,681,952 (Sifniades et al.) discusses a process for recovery of caprolactam from polycaprolactam-containing waste material. The process involves depolymerizing multi-component waste materials comprising polycaprolactam and non-polycaprolactam components to form caprolactam using superheated steam in the absence of an added catalyst. Superheated steam is fed though a distributor at the bottom of the reactor countercurrent to the flow of the melt of multi-component waste material. A vapor stream comprising caprolactam is collected at the top and condensed to obtain a condensate containing caprolactam. The caprolactam containing condensate may undergo distillation, crystallization and other conventional techniques in attempts to purify the caprolactam.
All of the processes heretofore for purifying impure caprolactam streams, whether the stream results from a caprolactam forming process or a depolymerization process, suffer from the same infirmities. The purification process is inefficient and costly to operate resulting in high energy usage, thus, raising the cost of caprolactam (which is traded as a commodity chemical). Therefore, an object of the present invention is to provide an efficient system for producing purified caprolactam.
A further object of the present invention is to provide a process for recovering purified caprolactam resulting from a depolymerization process.
Another object of the present invention is to provide a process for recovering purified caprolactam resulting from the depolymerization of polyamide-containing compositions.
Yet another object of the present invention is to provide a process for recovering purified caprolactam and medium-to-high pressure steam (usable steam).
A still further object of the present invention is to provide a caprolactam purification process whereby medium-to-high pressure steam recovered from crude caprolactam is utilized to assist the purification of the caprolactam-containing stream.
A further object of the present invention is to provide a caprolactam purification process for recovering purified caprolactam whereby the incoming caprolactam containing vapor stream is contacted with a quench fluid to minimize or prevent fouling of the distillation system with decomposition products.
The present invention relates to a process for achieving the above-noted objectives. The process involves purifying an impure stream of caprolactam while recovering not only pure caprolactam but also medium-to-high pressure steam. Preferably, the stream of impure or crude caprolactam, which comprises at a minimum a substantial portion of water, is obtained from depolymerization of a polyamide-containing composition. The resulting purified caprolactam will have a purity greater than 90 weight percent and the recovered steam will have a temperature sufficiently high to provide a driving force for use with other equipment, for instance, heat exchange equipment. One embodiment of the present invention provides a process for recovering caprolactam and usable steam comprising:
(a) providing a crude caprolactam feed stream comprising
(i) crude caprolactam including decomposition products, and
(ii) water and/or steam;
(b) if necessary, vaporizing the feed stream;
(c) optionally, introducing a quench fluid to remove at least a portion of the decomposition products from the vapor phase of the feed stream;
(d) distilling said vapor stream to recover medium-to-high pressure (usable) steam; and
(e) purifying said crude caprolactam to recover caprolactam.